1. Field of the Invention
The present invention relates to a method of manufacturing a color screen structure for a cathode-ray tube (hereinafter abbreviated "CRT").
2. Description of the Prior Art
In manufacturing a color screen structure for a color CRT, for example, a so-called aperture grill assembly as shown in FIG. 6 having a plurality of slits for passing electron beams therethrough includes an aperture grill member 9 formed of a thin metallic plate, such as a cold rolled thin steel plate and provided with a plurality of slits 6. The plate is placed on a metallic frame 5 consisting of a pair of opposite supporting members 1 and 2 and a pair of substantially U-shaped elastic connecting members 3 and 4 interconnecting the respective opposite ends of the supporting members 1 and 2. An external force is applied to the supporting members 1 and 2 of the frame 5 so as to stress the supporting members 1 and 2 toward each other by a fixed deflection, and a moderate tensile load is applied to the aperture grill member 9 to hold the aperture grill member 9 tight with respect to Y--Y direction. The aperture grill member 9 is welded by seam welding to the supporting members 1 and 2, the external force is then removed from the supporting members 1 and 2, and then the unnecessary peripheral portions 8 of the aperture grill member 9 are removed. The aperture grill member 9 is provided with slots 10 in the opposite sides thereof with respect to X--X directions outside the central region having the slits 6 and thin metallic strips 7. The respective upper surfaces of the supporting members 1 and 2 of the frame 5 in contact with the aperture grill member 9 have a radius of curvature Rz and the respective outer side surfaces of the supporting members 1 and 2 have a radius of curvature Ry.
In the conventional method of manufacturing such a color screening structure, the peripheral portions 8 of the aperture grill member 9 are removed by shearing the aperture grill member 9 with a roller cutter 11 as shown in FIG. 7. In cutting the aperture grill member 9, the reduced portion of the roller cutter 11 rolls along the curved surface of the aperture grill member 9 while the roller cutter 11 cuts off the peripheral portion 8 by shearing the aperture grill member 9 between the cutting edge thereof and the corner of the supporting frane 1 or 2 of the frame 5.
In shearing off the peripheral portions 8 of the aperture grill member 9 with the roller cutter 11, it is possible that metallic scraps, such as burrs 13 formed in the cut surface of the aperture grill member 9 as shown in FIG. 7 and/or chips 14, are produced due to an inappropriate combination of the size of the frame 5 and the roller cutter 11, the variation of the inclination 8 of the side walls of the supporting members 1 and 2 or the use of an abraded roller cutter. These metallic scraps 13 and 14 stick firmly to the aperture grill assembly, are unable to be removed completely from the aperture grill assembly by the subsequent washing and demagnetizing processes and, in the worst case, remain on the aperture grill assembly even after the aperture grill assembly has been placed within the vacuum interior of a CRT causing discharge, to deteriorate the voltage rating of the CRT. Thus the use of the roller cutter 11 for cutting off the peripheral portions 8 of the aperture grill member 9 produces metallic scraps entailing such problems
It is also possible to tear off the peripheral portions 8 along a welded seam 16 by taking advantage of the strength of the welded seam 16 as shown in FIG. 8 after seam-welding the aperture grill member 9 to the frame 5. This method of removing the peripheral portions 8 provides removal of the peripheral portions 8 without producing any metallic scrap. However, since the tearing force necessary for tearing off the peripheral portions 8 is proportional to the thickness of the aperture grill member 9, the greater the aperture grill member, the greater the tearing force will be. Furthermore, this method requires scrupulous attention to the control of the welded part and to the abrasion of the welding electrode roller to manufacture a color screening structure of satisfactory quality.